1. Field of the Invention
Recirculation breathing systems and more particularly a process and apparatus for supplying a variable and controlled composition of breathable gas mixture through cryogenic oxygen to a user and recirculating the exhaled gas through the cryogenic oxygen.
2. Background of the prior art
A large number of different types of equipment have been developed to maintain the oxygen content of a gaseous body that is being breathed by a human being. The types of equipment are commonly categorized according to the purpose to which the life support system is directed, including such categories for equipment as hard-hat deep-sea diving, skin diving, high altitude aerospace and submarine. Within each of these categories there has been developed different equipment to meet particular requirements of life-support, but each has the same design requirement to maximize the period during which a human being is supported while maintaining safety measures and economies.
Exposure to a gaseous body of pure oxygen is dangerous. There is danger of flash explosion or fire. The toxic effect of oxygen in high concentrations and pressures produces oxygen poisoning of the user. A deficiency of oxygen produces hypoxia which obviously can result in fatigue or death. Exposure to a gaseous mixture of oxygen and an inert gas or air poses little or no danger of flash explosion or fire, but the oxygen must be maintained at a certain concentration and pressure to avoid oxygen poisoning and hypoxia. Furthermore, the inert gas may have deleterious effects. Nitrogen at high pressures produces nitrogen narcosis and for that reason the most commonly used breathing mixture at high pressures is helium and oxygen.
Inert gases, particularly helium, are expensive. An open-circuit system in which the exhaled breathing mixture is vented requires large supplies of gas and heavy and expensive storage equipment which limits the support time, range, and manueverability of the user. The solution is a closed circuit recirculation system which reuses the uncomsumed gases. This self-contained system can be lighter and simpler, giving the user more range and manueverability. The exhaled gas must be treated to remove carbon dioxide and water vapor which if breathed in sufficient quantities are harmful. Also, for a given ambient pressure upon the user, the breathing gas mixture must have approximately an equal total pressure with the partial pressure of oxygen maintained at approximately 3 p.s.i.a., the optimum breathing partial pressure, for long periods of life-support. For a varying ambient pressure the composition of the breathing gas mixture must be varied correspondingly. In the present art composition variation requires complex and expensive equipment to analyze the gas composition and to remix the gas to the desired concentration.
The length of life-support time is limited by the amount of gas the self-contained apparatus can store. Cryogenic storage of the gas provides increased storage capacity for a limted volume and avoids the disadvantage of storage of the gas at high pressures.
In summary, the maximization of user manueverability and life-support time as well as safety and realization of economies can best be achieved by a recirculatory system employing a breathing gas mixture of an inert gas and oxygen in which the gas is cryogenically stored and the composition of the mixture is varied according to user requirements. Patented devices and processes employing some, but not all, of these characteristics include:
U.s. pat. No. 2,998,009 Breathing Apparatus PA1 U.s. pat. No. 3,016,053 Underwater Breathing Apparatus PA1 U.s. pat. No. 3,064,448 Air Conditioned Fuel Handling Suit PA1 U.s. pat. No. 3,366,107 Apparatus for Supplying Breathable Gas from Oxygen in Liquid Form
The present invention employs all of the characteristics described above and in a unique process. The oxygen is replaced and maintained at approximately the optimum breathing partial pressure of 3 p.s.i.a. by passing the breathing gas mixture through the liquid phase of a cryogenic liquid-vapor system containing oxygen. The cryogenic fluid is maintained in a two-phase condition to insure a gas head and liquid storage and to produce the oxygen replacement in the gas stream at saturation concentration so that the 3 p.s.i.a. partial pressure of the oxygen in the resultant mixture remains substantially constant and independent of the total pressure of the breathing gas mixture. To maintain the cryogenic fluid in the proper liquid-vapor form the system is maintained at a certain temperature and pressure. As the total pressure of the breathing gas mixture in increased additional inert gas is deposited to constitute a larger percentage of the mixture. The concentration of oxygen will be automatically decreased to hold the oxygen partial pressure at 3 p.s.i.a. This automatically controlled composition of breathing gas mixture is particularly suited for human breathing under a wide range of environmental pressure conditions. The gas mixture is heated to breathing temperature and continuously delivered to the user. Upon recirculation of the exhaled gas, carbon dioxide and water vapor within the exhaled gas are frozen out upon contact with the cryogenic and the remaining gas again is bubbled through the liquid phase of the liquid-vapor cryogenic system. According to this invention no special gas analyzing equipment is required to obtain desired concentration of oxygen in a breathing gas mixture having at least one inert gas mixed with oxygen. The composition of the breathing gas can be varied continuously to supply the desired total pressure of gas and partial pressure of oxygen to a user experiencing a wide range of ambient pressure variations. The manueverability, life support time, and safety of the user are maintained in a simple, efficient and inexpensive device embodying the invention.